This invention generally relates to photoresist materials useful in micro-lithography and, particularly, to improved materials and methods for pattern formation on semiconductor wafers.
Processes for patterning semiconductor wafers typically rely on lithographic transfer of a desired image from a thin-film of radiation-sensitive resist material. The process entails the formation of a sacrificial layer, the xe2x80x9cresistxe2x80x9d, which is photo-lithographically patterned. Generally these resists are referred to as xe2x80x9cphotoresistsxe2x80x9d.
The patterning of the resist involves several steps, including exposing the resist to a selected light source through a suitable mask to record a latent image of the mask and then developing and removing selected regions of the resist. For a xe2x80x9cpositivexe2x80x9d resist, the exposed regions are transformed to make such regions selectively removable; while for a xe2x80x9cnegativexe2x80x9d resist, the unexposed regions are more readily removable.
The pattern can be transferred into surface texture in the wafer by etching with a reactive gas using the remaining, patterned resist as a protective masking layer. Alternatively, when a wafer is xe2x80x9cmaskedxe2x80x9d by the resist pattern, it can be processed to form active electronic devices and circuits by deposition of conductive or semiconductive materials or by implantation of dopants.
Materials used in single layer photoresists for optical lithography should meet several objectives. Low optical density at the exposure wavelength and resistance to image transfer processes, such as plasma etching, are two important objectives to be met by such a photoresist material. Shorter wavelengths of radiation permit greater resolution. The most common wavelengths currently used in semiconductor lithography are 365 nm and 248 nm. The desire for narrower linewidths and greater resolution has sparked interest in photoresist materials that can be patterned by even shorter wavelengths of light.
The high absorbance of many organic functional groups at 157 nm makes it difficult to develop an organic polymer that is both base soluble and has low absorbance at 157 nm. Traditional photoresist polymers contain either phenols or carboxylic acids to solubilize the base polymer. Both organic groups, phenols and carboxylic acids, impart an excess of absorbance to the polymeric resist material to allow the polymer to be an effective component of a single layer photoresist for 157 lithography.
For example, many polymeric photoresists generally incorporate aromatic groups within or are appended to the polymer backbone currently used in UV lithography. Aromatic groups within these polymeric photoresists absorb energy at about 250 nm and absorb so much energy at sub 200 nm ranges that they are ill-suited for use at 157 nm. Generally, the currently used UV photoresists include phenolic groups to facilitate dissolution in basic aqueous developing solutions, and it is the phenolic groups which absorb energy at shorter wavelengths. One approach to improve dissolution of the photoresists has been to incorporate carboxylic acid groups within the polymeric structure of the photoresist. However, carbonyl groups absorb energy in the 160 nm range, thereby posing a difficult problem towards preparing photoresistive materials which can be used at or below the 157 nm range.
Photoresist materials and methods of photolithography at very short ultraviolet wavelengths are disclosed employing photosensitive compositions which include a photo-acid generator and an aliphatic polymer that includes one or more protected hydroxyl groups. In addition, one or more electron withdrawing groups attached to or adjacent to a hydroxyl containing carbon of the polymer can be incorporated to improve the base solubility of the polymer after removal of the hydroxyl protecting group. In one embodiment, the present invention provides positive photosensitive compositions which do not absorb, or minimally absorb, radiant energy at wavelengths greater than 157 nm, thereby improving performance characteristics for lithography applications. The photosensitive compositions of the invention are particularly useful at 157 nm with fluoride excimer lasers that emit radiation and provide enhanced resolution and imaging over currently known photoresistive compositions.
In one aspect of the invention, the aliphatic polymer component of the photoresist includes one or more electron withdrawing groups adjacent to, or attached to, carbon atoms bearing protected hydroxyl groups, and the protecting groups are labile in the presence of in situ generated acid. Suitable electron withdrawing groups are those which do not interact with, or minimally absorb, visible or near ultraviolet radiation include carboxylate ion, fluorine, chlorine, bromine, iodine, Nxe2x95x90NPh, carboxylic acid, carboxylic esters, e.g., t-butyl esters, ketone, trifluoromethyl, NH3+, CN, SO2Me, nitro, NMe3+, and N2+, preferably, fluorine or chlorine groups. In one embodiment, the protected hydroxyl groups are covalently attached to carbon atoms bearing these electron withdrawing groups. In one embodiment, the protected hydroxyl groups are adjacent to carbon atoms bearing electron withdrawing groups. In another embodiment, the protected hydroxyl groups are both covalently attached to carbon atoms bearing electron withdrawing groups and are adjacent to carbon atoms bearing electron withdrawing groups. In another embodiment, halogen atoms attached to or adjacent to a hydroxyl containing carbon of the polymer improve base solubility of the polymer after in situ generated acid removal of the hydroxyl protecting groups. In a preferred embodiment, the halogen atoms are fluorine atoms.
The present invention also provides methods to prepare such photosensitive compositions, and methods to prepare circuits and/or devices with the compositions.
An interaction between an energy source, e.g. a source that generates 157 nm radiation, and the photo-acid generator results in the release of acid which facilitates selective cleavage of protecting groups from hydroxyl sites. As a consequence, the resultant unprotected hydroxyl groups are susceptible to solubilization under basic conditions, i.e., the regions of the photoresist material that are exposed to the far UV radiation are rendered alkali soluble, whereas the unexposed (protected hydroxyl) regions of the photoresist material remain alkali insoluble. Suitable protecting groups for the hydroxyl groups of the polymer include acetals, ketals, esters (including carbonates) and ethers.
In one general class of positive photosensitive compositions, according to the invention, polymer components can have the formula 
where D is a carbon atom or a cyclic or bicyclic group. Y1, Y2 and Y3, are each independently hydrogen atoms, electron withdrawing groups (e.g., halogen atoms), or a pendent group as described below, and a is a positive value from 1 to 100, inclusive, b is a value from 0 to 100, inclusive, and z is a positive value from 2 to 100,000 inclusive. P is a protecting group for a hydroxyl group selected from the group of acetals, ketals, esters (including carbonates) and ethers and T denotes a covalent bond between carbon atom Da and OP, or is a bridging group having the formula: 
wherein Z1 and Z2 are each independently an electron withdrawing group or a hydrogen atom and f is a value from 0 to 6.
Each K, independently, is an electron withdrawing group, or a pendent group having the formula: 
where X1, X2, X3, X4 and X5 are each independently hydrogen atoms or electron withdrawing groups, preferably halogen atoms and g is a value from 0 to 4, inclusive.
For example, when D is a carbon atom and Y1, Y2 and Y3 are hydrogen atoms then at least one, and preferably at least two, of X1, X2, X3, X4 or X5 of T are electron withdrawing groups, e.g., halogen atoms, and when only one of Y1, Y2 and Y3 are halogen atoms, then at least one of X1, X2, X3, X4 and X5 of T preferably are electron withdrawing groups, e.g., halogen atoms. In some instances, when two or more of the X1-X5 constituents are electron withdrawing groups, it is preferably that both X1 and X2 or at least two of X3, X4 and X5 be electron withdrawing groups, e.g., when g is 0.
In another general class of positive photosensitive compositions, according to the invention, polymer components can have the formula 
where Y1, Y2 and Y3, are each independently hydrogen atoms, electron withdrawing groups, e.g., halogen atoms, or K, and a is a positive value from 1 to 100, inclusive, b is a value from 0 to 100, inclusive, and z is a positive value from 2 to 100,000 inclusive. P is a protecting group for a hydroxyl group selected from the group of acetals, ketals, esters (including carbonates) and ethers and T denotes a covalent bond between carbon atom Ca and OP, or is a bridging group having the formula: 
wherein Z1 and Z1 are each independently an electron withdrawing group or a hydrogen atom and f is a value from 0 to 6.
Each K, independently, is an electron withdrawing group, or a pendent group having the formula: 
where X1, X2, X3, X4 and X5 are each independently hydrogen atoms or electron withdrawing groups, preferably halogen atoms and g is a value from 0 to 4, inclusive. When Y1, Y2 and Y3 are hydrogen atoms then at least one, and preferably at least two, of X1, X2, X3, X4 or X5 of T are electron withdrawing groups, e.g., halogen atoms, and when only one of Y1, Y2 and Y3 are halogen atoms, then at least one of X1, X2, X3, X4 and X5 of T preferably are electron withdrawing groups, e.g., halogen atoms. In some instances, when two or more of the X1-X5 constituents are electron withdrawing groups, it is preferably that both X1 and X2 or at least two of X3, X4 and X5 be electron withdrawing groups, e.g., when g is 0.
In another embodiment, the polymer has the formula 
where J is a cyclic or bicyclic group and where Y2 and Y3, are each independently hydrogen atoms, electron withdrawing groups, e.g., halogen atoms, or K, and a is a positive value from 1 to 100, inclusive, b is a value from 0 to 100, inclusive, and z is a positive value from 2 to 100,000 inclusive. P is a protecting group for a hydroxyl group and T is a bridging group having the formula: 
wherein Z1 and Z2 are each independently an electron withdrawing group or a hydrogen atom and f is a value from 0 to 6.
Each K, independently, is an electron withdrawing group, or a pendent group having the formula: 
where X1, X2, X3, X4 and X5 are each independently hydrogen atoms or electron withdrawing groups, preferably halogen atoms and g is a value from 0 to 4, inclusive.
Suitable cyclic and bicyclic groups for J include, but are not limited to, cyclohexyl groups, cyclopentyl, cycloheptyl, and norbornyl. Therefore, suitable monomers include those which include at least one degree of unsaturation within the cyclic or bicyclic structure such that polymerization can occur between reactive monomers.
It should be understood that not all hydroxyl groups need to be protected, that is, the hyroxyl groups within the polymer matrix can be partially protected. In general, it is considered advantageous to have at least 10% of the hydroxyl groups protected in a hydroxyl containing homopolymer. Preferably, between about 15% and about 50% of the hydroxyl groups are protected in the homopolymer.
Copolymers (including ter-, tetra-, etc. polymers) that contain unprotected hydroxyl groups are useful in the present invention. In one embodiment, at least 10% of the hydroxyl groups are protected. In another embodiment, at least 15% of the hydroxyl groups are protected. Again, preferably, between about 15% and about 50% of the hydroxyl groups are protected.
It should also be understood, and is well known by those skilled in the art, that most of the formulations suitable for use in this invention contain a small amount of base which may help to stabilize the polymer system. In general, less than 1% is a base component, based on the total weight of the polymer composition, e.g., less than 0.5%. Suitable bases typically are organic bases known in the art such as tetrabutylammonium hyroxide, diazabicyclo[5.4.0]undec-7-ene, diphenyl amine, trioctyl amine, or triheptal amine.
Suitable photo-acid generators include onium salts, such as triphenylsulfonium salts, sulfonium salts, iodonium salts, diazonium salts and ammonium salts, 2,6-nitrobenzylesters, 1,2,3-tri(methanesulfonyloxy)benzene, sulfosuccinimides and photosensitive organic halogen compounds as disclosed in Japanese Examined Patent Publication No. 23574/1979.
In addition to the hydroxyl protected polymer and photo-acid generator, small molecules which help to inhibit hydrolysis of the protected hydroxyl groups can be included in the compositions of the invention. These small molecules are typically ester, ether, ketal or acetal protected low molecular weight polyhydridic alcohols or low molecular weight alcohols. The protecting groups can further include those listed below. Suitable low molecular weight hydrolysis inhibitors include, for example, Di-Boc Bisphenol A, Di-Boc o-cresolphthalein, tert-butyl lithocholate and tert-butyl deoxycholate (available from Midori Kagaku Col, Ltd. Tokyo, Japan).
Thus the above described compositions include protected hydroxyl groups which are labile in the presence of in situ generated acid. Upon exposure to a far UV energy source, e.g. a source which generates 157 nm radiation, the photo-acid generator will release acid to facilitate selective cleavage of protecting groups from protected hydroxyl sites. As a consequence, the resultant unprotected hydroxyl groups will be susceptible to solubilization under basic conditions and the exposed photoresist material is rendered alkali soluble, whereas the unexposed photoresist material will remain alkali insoluble.
Other advantages of the invention will be readily apparent to one having ordinary skill in the art upon reading the following description.
All percentages by weight identified herein are based on the total weight of the polymer composition unless otherwise indicated.